Space-variant polarization manipulation of thermal emission

ABSTRACT

A method for Space-variant polarization manipulation of enhanced nondirectional thermal emission in a narrow spectral peak is disclosed, comprising providing a subwavelength grating irradiating non-directional thermal emission on the grating and discretely controlling the local orientation of the grating.

FIELD OF THE INVENTION

The present invention relates to thermal emission. More particularly itrelates to space-variant polarization manipulation of thermal emissionobtained with subwavelength grating supporting surfacephonon-polarization or surface plasnom-polarization.

BACKGROUND OF THE INVENTION

Thermal emission from absorbing material is considered to be incoherentand unpolarized, and accordingly is regarded as spontaneous emission.The surface properties of the absorbing material have a profound impacton the emission's optical properties, and can be manipulated to producea partially coherent and partially polarized radiation emission.Recently, it was shown that by etching a uniform grating on a SiCsubstrate, a highly directional peak of thermal emission was achieved.Furthermore, spectral resonance and nondirectional emission wereobserved at certain frequencies. In these instances, a connectionbetween the emission and the surface properties was established bystudying the excitation of surface phonon-polaritons (SPPs). Theunderlying microscopic origin of the SPP is the mechanical vibration ofthe atoms. A surface polariton (phonon or plasmon) has a longer wavevector than the light waves propagating along the surface with the samefrequency. For this reason, they are called “nonradiative” surfacepolaritons. By coupling the surface polaritons with the propagating waveby means of an additional prism or grating, one can produce eitherincreased resonant absorption or directional emission. Because SPPs orsurface plasmon-polaritons are able to be excited only by TM-polarizedpropagating waves, the emission's characteristics have to bepolarization-dependent. The TM polarization state has an electric-fieldcomponent that is parallel to the grating vector (see inset in FIG. 1(a) for TE and TM definitions).

SUMMARY OF THE PRESENT INVENTION

There is thus provided, in accordance with some preferred embodiments ofthe present invention a method for Space-variant polarizationmanipulation of enhanced nondirectional thermal emission in a narrowspectral peak comprising providing a subwavelength grating irradiatingnon-directional thermal emission on the grating and discretelycontrolling the local orientation of the grating.

Furthermore, in accordance with some preferred embodiments of thepresent invention, the thermal emission is in infrared range.

Furthermore, in accordance with some preferred embodiments of thepresent invention, the method further comprises providing a thermalimaging sensor for imaging the thermal emission.

Furthermore, in accordance with some preferred embodiments of thepresent invention, the method further comprises designing the spatialorientation of the grating with a random key for optical encryption ofinformation.

Furthermore, in accordance with some preferred embodiments of thepresent invention, the method further comprises decrypting the encryptedinformation using an imaging sensor for obtaining image datacorresponding to the thermal emission off the grating and processing theimage data using a correct key.

Furthermore, in accordance with some preferred embodiments of thepresent invention, the method is used for spatially modulated heattransfer.

Furthermore, in accordance with some preferred embodiments of thepresent invention, the method is used for formation of high efficiencythermal sources.

Furthermore, in accordance with some preferred embodiments of thepresent invention, the grating is provided on material selected frompolar materials in a spectral range where ∈′<−1, where ∈′ is the realpart of the dielectric constant of the polar material.

Furthermore, in accordance with some preferred embodiments of thepresent invention, the grating is provided on a substrate made from aconductive material.

Furthermore, in accordance with some preferred embodiments of thepresent invention, the grating is provided on a substrate made from adielectric material.

Furthermore, in accordance with some preferred embodiments of thepresent invention, the grating is provided on a substrate made fromfused silica.

Furthermore, in accordance with some preferred embodiments of thepresent invention, surface plasmon polaritons are excited.

Furthermore, in accordance with some preferred embodiments of thepresent invention, the grating comprises spiral elements.

Furthermore, in accordance with some preferred embodiments of thepresent invention, the spiral elements have a discrete local grooveorientation of φ=mω/2, where m is the polarization order and ∈ is theazimuthal angle of the polar coordinates.

BRIEF DESCRIPTION OF THE FIGURES

In order to better understand the present invention, and appreciate itspractical applications, the following Figures are provided andreferenced hereafter. It should be noted that the Figures are given asexamples only and in no way limit the scope of the invention. Likecomponents are denoted by like reference numerals.

FIG. 1 illustrates in (a) The calculated SiO₂ spectral reflectance innormal direction for a flat surface (crosses), in (b) the spectraldependence of the real part of the SiO₂ dielectric constant, ∈′, in (c)the calculated emissivity vs. grating depth for TM and TE polarizations,in (d) the calculated emissivity vs. grating fill factor (q) for TM andTE polarizations.

FIG. 2 illustrates measured and calculated spectral reflectance with anincidence angle of θ=20° of a (a) flat surface (b) uniform SiO₂ grating.

FIG. 3 illustrates measured (solid) and calculated (dashed) relativeemissivity spectrum of the grating for TM, TE and total (without apolarizer) emission in (a) normal observation direction θ=0° and (b) inθ=30°.

FIG. 4 illustrates in (a) a scanning electron microscope (SEM) image ofthe spiral subwavelength elements with polarization order numbers m=1,2, 3 and 4. Thermal emission images emerging from the SiO₂ spiralelements (b) captured through a polarizer, and (c) without a polarizer,for m=1, 2, 3, 4.

FIG. 5 illustrates in (a) image intensity to be encrypted, in (b) thecorrect key function in gray level composite of 20×20 pixels array, in(c) a SEM image of four pixels of the encrypted element, in (d)intensity picture of the encrypted element through a linear polarizertalken by a thermal camera, in (e) decrypted image achieved bydecryption process calculating Stokes parameters applying intensitiespictures and the correct key, in (f) wrong key function in gray level,and in (g) decrypted image resulted from using the wrong key.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

We introduce a theoretical and experimental investigation ofspace-variant polarization-dependent thermal emission by exploiting thepolarization dependence of the SPPs (or surface plasmon-polaritons) indifferent material configuration. Computer-generated subwavelengthgratings etched on fused silica (SiO₂) substrates are used to generatespace-variant polarization radiation. As a first step, we designed agrating to enhance the nondirectional thermal emission to form a narrowspectral peak for TM polarization. We were then able to experimentallydemonstrate space-variant polarization manipulation of thermal emissionby discretely controlling the local orientation of the grating. To thebest of our knowledge, this was the first time that space-variantpolarization manipulation of infrared thermal emission had beenachieved. This phenomenon can be exploited in a variety of applicationssuch as thermal polarization imaging, optical encryption, spatiallymodulated heat transfer and the formation of high efficiency thermalsources.

SPPs are supported by polar materials in the spectral range where ∈′<−1(∈′ is the real part of the dielectric constant). There are two kinds ofmaterials that support surface waves: conductive materials that supportsurface plasmon-polaritons, and dielectric materials that support SPPs.As can be seen in FIG. 1( b), fused silica behaves as a polar materialin the spectral range of 8.65 μm to 9.18 μm. Our goal was to design agrating on a SiO₂ substrate for which nondirectional emission wasrestricted to a narrow spectral band. In opaque materials the emissivity(∈) is related to the reflectance (R) via Kirchhoff's law, ∈=1−R foreach direction, wavelength, temperature and polarization. In order tomaximize the emissivity, we optimized the SiO₂ grating using a spectralreflectance calculation by rigorous coupled wave analysis (RCWA). FIG.1( c) shows the calculation of the emissivity as a function of thegrating depth for normal incident light with a wavelength of 8.93 μm.There is a strong variation in the emissivity as a function of thegrating depth only for TM polarization. The dependence of the emissivityas a function of the grating's fill factor (q) was also calculated withthe previous parameters, but with a grating depth of 0.7 μm as shown inFIG. 1( d). The optimal grating parameters were determined to be: periodΛ=2 μm, fill factor q=0.5 and grating depth h=0.7 μm.

FIG. 1( a) shows the calculated spectral reflectance of the grating forTE and TM polarization states as well as that of the flat surface fornormal incident light. Note that for λ=8.93 μm, the TE reflectioncoincides with the reflectance of the flat surface, while the TMreflection is close to zero. We ascribe the spectral resonance of thereflectance to the excitation of SPPs. According to Kirchhoff's law, weexpected to obtain a high discrimination between the emissivity of theTE and TM polarizations. As a next step we defined the emissivitymodulation to be η=|(∈_(TM)−∈_(TE))/(∈_(TM)+∈_(TE))|, where ∈_(TM) and∈_(TE) are the emissivity values for the TM and TE polarization states,respectively. The optimized grating parameters cited above yielded ahigh emissivity modulation of η=0.52 for angles up to 30°.

In order to confirm our theoretical predictions, we formed a 10 mm×10 mmuniform grating on an amorphous SiO₂ substrate using advancedphotolithographic techniques. A Cr film was deposited on a SiO₂substrate and overcoated with a positive photoresist. After exposing thephotoresist through a mask, it was developed leaving a strip pattern onthe Cr film. A Cr etchant was then applied to remove the Cr film fromthe exposed areas. At this point the photoresist was removed and thesubstrate etched by reactive ion etching (RIE) through the Cr strips,which served as a mask. The RIE was performed at a power of 175 W and apressure of 40 mTorr with CF₄ and O₂ gas flow rates of 13.8 and 1.2sccm, respectively. The etching, performed at a rate of 35 Å per minuteat room temperature, was continued until the desired depth was reached.As a final step the remaining Cr was removed with a Cr etchant.

The inset in FIG. 2( b) shows a scanning electron microscope (SEM) imageof the grating. Due to inaccuracies in fabrication, the actual fillfactor was 0.3 instead of 0.5. For this fill factor the optimal depthwas determined to be 0.8 μm instead of 0.7 μm. We began by illuminatingthe grating with an infrared source (SiC 1270° K, SP-Oriel 80007) at anincidence angle of 20°. We measured the reflectance for bothpolarization states with an infrared Fourier transform spectrometer(FTIR, SP-Oriel MIR 8000, resolution 4 cm⁻¹) equipped with a cooledHgCdTe detector (SP-Oriel 80026). FIG. 2 shows the measured and thecalculated spectral reflectance values at 20° incidence to a flatsurface and to the grating. The results are in good agreement with thecalculated values for both polarizations. For these grating parameterswe obtained an emissivity modulation of η=0.33.

Spectral measurements of the emissivity were then performed by use ofFTIR. In this experiment, the sample was heated to 873° K with aprecision better than 1° K (heater and temperature-controller fromHeatWave Labs Inc.). FIG. 3 shows the measured and calculated spectraldependence of the relative emissivity for TM and TE polarizations, aswell as without a polarizer, in a normal emission direction and forη=300. The relative emissivity is defined as the grating emissivity(∈_(G)) normalized to the emissivity of the flat surface (∈_(F)) foreach case. A narrow spectral peak of Δλ=90 nm was obtained for TMpolarization around a wavelength of 9.07 μm. Its relative emissivity was2.75, while the relative emissivity of TE polarization was approximatelyunity. The measured peak wavelength of the relative emissivity wasshifted with respect to the predicted value. This results fromtemperature-related variations in the dielectric constant. The inset inFIG. 3( a) shows both experimental and calculated relative emissivity asa function of the emission angle, and indicates that the peak emissivitywas nondirectional. Coupling of the emission in any direction ispossible if the SPP dispersion relation is flat.

Finally, in order to demonstrate space-variant polarization-dependentthermal emission, we formed four space-variant spiral elements having adiscrete local groove orientation of φ=mω/2, where m is the polarizationorder and ω is the azimuthal angle of the polar coordinates. Theelements were 10 mm in diameter with 16 discrete levels and designed forpolarization order numbers of m=1, 2, 3 and 4. SEM images of the centralarea of the elements are shown in FIG. 4( a). FIG. 4( b) shows thespatial thermal emission distributions after emerging from the spiralelements at 353° K, then passed through a linear polarizer and capturedby a thermal camera (CEDIP, 320×240 pixels). Space-variant spiral-likeintensity modulation, resulting from the space-variantpolarization-dependent emissivity, is clearly observed. The distributionof the emissions from the spiral elements not passed through a polarizeris shown in FIG. 4( c) in which the black lines indicate the local TMpolarization orientation. In this case, the emission distribution isalmost uniform. However, an axially symmetric polarization orientationis obtained in the near-field for the enhanced TM emission. As expectedfrom FIG. 3, the total intensity emitted from the grating is higher thanfrom the flat surface emission due to the enhanced TM emission.

In this section, we briefly present a novel approach for opticalencryption by using the polarization dependence of thermal emissionsupporting SPPs or surface plasmon-polaritons. Computer-generatedsubwavelength grating etched on fused silica (SiO₂) substrate is used togenerate space-variant polarization radiation. As we have shown, theorientation of the local grating relative to the orientation of thepolarizer determines the intensity detected by the camera. Let us havean image, as shown in FIG. 5( a), which has to be encrypted with arandom key (for example, FIG. 5( b)). By designing the spatialorientation of the gratings we produced an encrypted image. A magnifiedarea of the element by SEM is shown in FIG. 5( c). A thermal camera isused to capture the emerging radiation through a linear polarizer. Theintensity picture in FIG. 5( d) is obtained when the polarizer is sitedto zero. The decryption process was done by software using the correctkey. The decrypted image shown in FIG. 5( e) was attained by calculatingthe Stokes parameters when applying the intensities, and applying thecorrect key. A case in which the wrong key is used, as depicted in FIG.5( f), the resulting decrypted image would show only white noise as canbe seen in FIG. 5( g), with no possibility of reconstructing theoriginal image. To the best of our knowledge, this is the first timethat optical encryption based on thermal emission supporting SPPs hasbeen achieved.

In conclusion, we have demonstrated a narrow spectral relativeemissivity peak for a broad range of observations for a SiO₂ grating.The enhanced thermal infrared radiation, which was obtained only with TMpolarization, was attributed to the excitation of SPPs. In the case ofinterface between conductive and dielectric materials the enhanceemission is attributed to surface plasmon-polaritons. Using thepolarization dependence of the emissivity, a space-variant polarizationmanipulation of the thermal emission was experimentally demonstrated bycontrolling the local orientation of the subwavelength grating.

Reference is now made to the figures.

FIG. 1. (a) The calculated SiO₂ spectral reflectance in normal directionfor a flat surface (crosses), and for a grating with period Λ=2 μm, fillfactor q=0.5 and depth h=0.7 μm for TM polarization (triangles) and TEpolarization (circles). The inset illustrates the illumination scheme ofthe grating. (b) The spectral dependence of the real part of the SiO₂dielectric constant, ∈′. (c) The calculated emissivity vs. grating depthfor TM and TE polarizations for a wavelength of 8.93 μm in normaldirection of light, with grating parameters of: period Λ=2 μm, fillfactor q=0.5. (d) The calculated emissivity vs. grating fill factor (q)for TM and TE polarizations for a wavelength of 8.93 μm in normaldirection of light, with grating parameters of: period Λ=2 μm, depth=0.7μm.

FIG. 2. Measured and calculated spectral reflectance with an incidenceangle of θ=20° of a (a) flat surface (b) uniform SiO₂ grating. Thegrating parameters of: Λ=2 μm, fill factor q=0.3 and depth h=0.8 μm; theexperimental results for TE and TM (solid lines), calculated results forTE (dashed line with circles) and for TM polarization (dashed line withtriangles). The inset shows a scanning electron microscope (SEM) imageof the grating.

FIG. 3. Measured (solid) and calculated (dashed) relative emissivityspectrum of the grating for TM, TE and total (without a polarizer)emission in (a) normal observation direction θ=0° and (b) in θ=30°.Inset shows the measured and calculated (solid lines) relativeemissivity as a function of observation angle for TM (triangle) and TE(circle) polarization.

FIG. 4. (a) A scanning electron microscope (SEM) image of the spiralsubwavelength elements with polarization order numbers m=1, 2, 3 and 4.Thermal emission images emerging from the SiO₂ spiral elements (b)captured through a polarizer, and (c) without a polarizer, for m=1, 2,3, 4. The elements were uniformly heated to a temperature of 353° K. Thelines indicate the local TM polarization orientation measured in thenear-field.

FIG. 5. (a) Image intensity to be encrypted. (b) The correct keyfunction in gray level composite of 20×20 pixels array. (c) A SEM imageof four pixels of the encrypted element. (d) Intensity picture of theencrypted element through a linear polarizer taken by a thermal camera.(e) Decrypted image achieved by decryption process calculating Stokesparameters applying intensities pictures and the correct key. (f) Wrongkey function in gray level. (g) Decrypted image resulted from using thewrong key.

To conclude, space-variant polarization manipulation of enhancednondirectional thermal emission in a narrow spectral peak was presentedhereinabove. The emission is attributed to surface phonon-polaritonexcitation from space-variant subwavelength SiO₂ gratings, or surfaceplasmon-polaritons excitation from metal-dielectric interface.Polarization manipulation was obtained by discretely controlling thelocal orientation of the grating. We experimentally demonstrated thermalemission in an axially symmetric polarization distribution. Theoreticalcalculations based on rigorous coupled-wave analysis are presented alongwith experimental results.

It should be clear that the description of the embodiments and attachedFigures set forth in this specification serves only for a betterunderstanding of the invention, without limiting its scope.

It should also be clear that a person skilled in the art, after readingthe present specification could make adjustments or amendments to theattached Figures and above described embodiments that would still becovered by the present invention.

1. A method for Space-variant polarization manipulation of enhancednondirectional thermal emission in a narrow spectral peak comprisingproviding a subwavelength grating irradiating non-directional thermalemission on the grating and discretely controlling the local orientationof the grating.
 2. The method of claim 1, wherein the thermal emissionis in infrared range.
 3. The method of claim 1, further comprisingproviding a thermal imaging sensor for imaging the thermal emission. 4.The method of claim 1, further comprising designing the spatialorientation of the grating with a random key for optical encryption ofinformation.
 5. The method of claim 4, further comprising decrypting theencrypted information using an imaging sensor for obtaining image datacorresponding to the thermal emission off the grating and processing theimage data using a correct key.
 6. The method of claim 1, used forspatially modulated heat transfer.
 7. The method of claim 1, used forformation of high efficiency thermal sources.
 8. The method of claim 1,wherein the grating is provided on material selected from polarmaterials in a spectral range where ∈′<−1, where ∈′ is the real part ofthe dielectric constant of the polar material.
 9. The method of claim 1,wherein the grating is provided on a substrate made from a conductivematerial.
 10. The method of claim 1, wherein the grating is provided ona substrate made from a dielectric material.
 11. The method of claim 1,wherein the grating is provided on a substrate made from fused silica.12. The method of claim 1, wherein surface plasmon polaritons areexcited.
 13. The method of claim 1, wherein the grating comprises spiralelements.
 14. The method of claim 13, wherein the spiral elements have adiscrete local groove orientation of φ=mω/2, where m is the polarizationorder and ω is the azimuthal angle of the polar coordinates.